1. Field of the Invention
When a cable, rope, hose or other long, thin member is suspended in air or another fluid like water, and there is a component of relative movement between the side of the cable and the fluid, there is a drag force due to the dynamic pressure created when the otherwise undisturbed streamlines of flow in the fluid are diverted to circumvent the cable. The suspended member, such as a cable which has a round cross-section, has stagnation points of flow both in front of and behind the cable, and is not ideally suited to allowing the fluid to flow by with minimum drag. The force due to an upstream stagnation point is not balanced by a force at the downstream stagnation point, and so drag occurs. The unbalanced drag force is proportional to fluid density and the square of the relative velocity for modest velocities.
Such a cable typically is held in tension in the fluid. Reducing the drag decreases the tension force on the cable and the stress in the supporting structure, so that both the cable and supports can be made lighter in weight, more reliable and less costly.
2. Description of the Prior Art
An Information Disclosure Statement accompanies this application.
Streamlining in fluid flow has been applied to long, thin members such as the struts on hydrofoils and older aircraft and to the guy wires used between the wings of biplanes, always by shaping the member itself to minimize the drag profile presented to the flow. The flow always came from the same direction, and so the streamlining did not need to, nor could it adapt to a changing flow direction.
Wiener (U.S. Pat. No. 2,859,836, 1958) discloses adding tear-drop shaped pieces to cables but strictly for the purpose of suppressing vibrations. The present invention avoids the vibration problem by using short pieces having a streamlined shape which are independently rotatable to prevent vibration modes.
Wiener does not claim a drag-reduction property, which is the main feature on this invention. Further, Wiener's pieces must be threaded onto the cable from the end before the cable is installed, whereas the present streamlining elements can be applied to a cable in place. Each of Weiner's pieces could rotate on the cable, though the pieces were clearly separated at isolated points along the cable. In the present invention contiguous independently rotatable elements surround the cable over the whole immersed length.
Wiener claims a blunt forward end which, when a mass is added, serves his invention by moving the center of gravity forward of the center of rotation to control vibration, whereas in the present invention these positions are reversed. In addition, the present invention specifically includes a pointed upstream end which acts functionally in a very different manner.